Stereo To Surround Sound Conversion

This question may have been answered in the past, but I can't find it if it was. I want to know if there are any free programs out there capable of converting 2 channel (stereo) audio into 5.1 audio. I just bought a 5.1 sound system and put in a 5.1 sound card and it seems like a major waste to only be using 2 of my speakers.

Thanks!